Environmental Water Treatment Knowledge: The Meaning and Function of Electrodialysis

Electrodialysis is a combination of an electrochemical process and a dialysis-diffusion process. Driven by an external direct current electric field and utilizing the selective permeability of ion-exchange membranes (i.e., cations can pass through the cation-exchange membrane and anions can pass through the anion-exchange membrane), cations and anions migrate toward the anode and cathode respectively. During ion migration, if the fixed charge of the membrane is opposite to the ion's charge, the ion can pass; if their charges are the same, the ion is repelled, thereby achieving the purposes of desalination, concentration, refining, or purification of the solution [1].

Compared with reverse osmosis, another membrane separation technology introduced in recent years, electrodialysis is cheaper but has a lower desalination rate. When

domestic ion-exchange membranes are also of very stable quality and the operation and management are very convenient.

The semipermeable membrane used in electrodialysis is actually an ion-exchange membrane. According to the charge nature of the ions, this ion-exchange membrane is divided into two types: cation-exchange membrane (cationic membrane) and anion-exchange membrane (anionic membrane). In an electrolyte aqueous solution, the cationic membrane allows cations to pass through while repelling and blocking anions, and the anionic membrane allows anions to pass through while repelling and blocking cations; this is the selective permeability of the ion-exchange membrane. During electrodialysis, the ion-exchange membrane does not exchange with a certain ion in the aqueous solution as an ion-exchange resin does, but only plays a selective permeation role for ions of different charge nature, i.e., the ion-exchange membrane does not need regeneration. The compartment composed of the electrodes and membranes in the electrodialysis process is called the electrode chamber, where the electrochemical reactions occurring are the same as ordinary electrode reactions. Oxidation occurs in the anode chamber, the anode water becomes acidic, and the anode itself is easily corroded. Reduction occurs in the cathode chamber, the cathode water becomes alkaline, and scale easily forms on the cathode [2].

Electrodialysis is a relatively mature membrane separation technology, widely used for brackish water desalination and is a major method of producing fresh water in some regions of the world. Because newly developed charged membranes have higher selectivity, lower membrane resistance, better thermal and chemical stability, and higher mechanical strength, the electrodialysis process is not limited to desalination but also has many other industrial applications in the food, pharmaceutical, and chemical industries, such as industrial wastewater treatment, mainly including recovering acid and metal from the waste liquid formed by acid cleaning of metal surfaces; recovering heavy metal ions from electroplating wastewater; recovering sulfate from synthetic fiber wastewater; and recovering sulfite from pulp waste liquid. In the food industry it is used for milk desalination to make infant formula; in the chemical industry for separating ionic from non-ionic substances; and in clinical treatment electrodialysis can be used as an artificial kidney, etc. [3]

Automatically controlled electrodialysis with frequent polarity reversal (EDR) makes operation and management more convenient. The raw water utilization rate can reach 80%, and the general raw water recovery rate is between 45% and 70%. Electrodialysis is mainly used for primary desalination of water, with a desalination rate between 45% and 90%. It is widely used for desalination of seawater and brackish water; primary desalination in the preparation of pure water; and desalination and softening of boiler and power equipment feed water, etc.

Essentially, electrodialysis can be said to be a desalting technology, because all kinds of water (including natural water, tap water, and industrial wastewater) contain a certain amount of salt, and the anions and cations that make up these salts will move toward electrodes in opposite directions under the action of a direct current electric field. If one cation-exchange membrane and one anion-exchange membrane are inserted into an electrodialyzer, because the ion-exchange membranes have selective permeability-i.e., the cation-exchange membrane only allows cations to pass freely and the anion-exchange membrane only allows anions to pass-the salt concentration in the middle compartment between the two membranes will decrease due to the directional migration of ions, while the two compartments near the electrodes become concentration chambers for anions and cations respectively, finally achieving desalination in the middle desalination compartment.

In practical applications, an electrodialyzer is not composed of just one pair of anion- and cation-exchange membranes (because this would be very inefficient), but uses one hundred or even several hundred pairs of membranes, thereby greatly improving efficiency.

At present, electrodialyzers have a wide range of applications: in water desalination and desalting, seawater concentration for salt production, refining dairy products, debittering and purifying fruit juice, and producing chemical products, etc.; they can also be used in the food and light industries for producing pure water and as pre-treatment for producing high-purity water in the electronics and pharmaceutical industries; and for primary softening and desalting of boiler feed water and desalting brackish water into drinking water.

Electrodialyzers are suitable for water treatment in the electronics, pharmaceutical, chemical, thermal power generation, food, beer, beverage, printing and dyeing, and coating industries. They can also be used for physical and chemical processes such as concentration, purification, and separation of materials.

Electrodialysis can also be used for the treatment of wastewater and waste liquids and the recovery of precious metals, such as recovering nickel from electroplating waste liquid.

(1) Operating pressure around 0.5-3.0 kg/cm2

(2) Operating voltage and current: 100-250 V, 1-3 A

(3) Power consumption of the unit: about 0.2-2.0 kWh per ton of fresh water

(6) Water pressure permeation: because there is a fluid pressure difference between the concentration chamber and the desalination chamber, water molecules are forced to permeate from the high-pressure side to the low-pressure side. Obviously, these secondary processes are unfavorable factors for electrodialysis, but they can all be avoided or controlled by changing operating conditions.

(2) It can be used for the purification of non-electrolytes such as sucrose to remove the electrolytes therein;

(3) In principle, an electrodialyzer is an electrolytic cell with a diaphragm, and it can make efficient use of the redox reactions on the electrodes.

IV. During the electrodialysis process, the following secondary processes also take place

(1) Migration of co-ions: the selective permeability of the ion-exchange membrane can never be 100%, so a small amount of counter-ions always passes through the exchange membrane;

(2) Concentration diffusion of ions: because there is a concentration difference between the solutions in the concentration chamber and the desalination chamber, a small amount of ions always diffuses and migrates from the concentration chamber to the desalination chamber, thereby reducing the dialysis efficiency;

(3) Water permeation: although the exchange membrane does not allow solvent molecules to pass through, because there is a concentration difference between the desalination chamber and the concentration chamber, some solvent molecules (water) will permeate toward the concentration chamber;

(4) Electro-osmosis of water: due to the hydration of ions and the formation of an electric double layer, under the action of a direct current electric field, water molecules can also migrate from the desalination chamber to the concentration chamber;

(5) Polarization ionization of water: sometimes, due to poor working conditions, water is forced to ionize into hydrogen ions and hydroxide ions, which can pass through the exchange membrane into the concentration chamber;

(1) It can simultaneously desalinate, concentrate, separate, and purify an electrolyte aqueous solution;